1. Field of the Invention
This invention relates to a control system for plants, which controls a plant by means of feedback control based on an adaptive control theory.
2. Prior Art
Conventionally, an air-fuel ratio control system for internal combustion engines is known, e.g. from U.S. Pat. No. 5,636,621, in which a parameter-adjusting mechanism calculates adaptive parameters and an adaptive controller carries out feedback control of the air-fuel ratio of a mixture supplied to the engine to a desired air-fuel ratio by using the adaptive parameters. In the known air-fuel ratio control system, an air-fuel ratio sensor arranged in the exhaust system of the engine detects the air-fuel ratio of the mixture and supplies a signal indicative of the detected air-fuel ratio to the adaptive controller, which in turn determines an amount of fuel to be supplied to the engine such that the detected air-fuel ratio becomes equal to a desired air-fuel ratio. In this case, the internal combustion engine is a plant as an object to be controlled, while the amount of fuel to be supplied to the engine and the desired air-fuel ratio are a manipulated variable and a desired value to which an operation of the plant is to be controlled, respectively.
In the air-fuel ratio control system disclosed in the above publication, to perform the adaptive control in a stable manner, limit-checking of an identification error signal (indicative of a degree of deviation of the adaptive parameters from their optimal values) for use in calculation of the adaptive parameters by the adaptive parameter-adjusting mechanism is carried out such that the value of the signal falls within a predetermined range defined by upper and lower limit values.
However, even if the value of the identification error signal is within the predetermined range, calculation of the adaptive parameter vector .theta.(k) is made by adding the product of the identification error signal e* multiplied by a predetermined gain to the immediately preceding value .theta.(k-1), as shown in Mathematical Formula (8) in the above publication. Accordingly, the identification error signal e* is accumulated in the adaptive parameter vector .theta. through the multiplication. Therefore, in the conventional control system, when the adaptive control is continued while the engine as the object to be controlled is in a steady and stable operating condition, it cannot be avoided that the adaptive parameter vector .theta. drifts (progressively deviates from the optimal value).